Available forms

Tibolone is available in the form of 2.5 mg oraltablets.[28] It is typically used once daily at a dosage of 1.25 or 2.5 mg.[28]

Side effects

A report in September 2009 from Health and Human Services' Agency for Healthcare Research and Quality suggests that tamoxifen, raloxifene, and tibolone used to reduce the risk of breast cancer significantly reduce the occurrence of invasive breast cancer in midlife and older women, but also increase the risk of adverse effects.[29]

It was reported in 2002 that tibolone or its metabolite δ4-tibolone is transformed by aromatase into the potent estrogen 7α-methylethinylestradiol in women, analogously to the transformation of norethisterone into ethinylestradiol.[34] Controversy and disagreement followed when other researchers contested the findings however.[35][36][37][38][39][40] By 2008, these researchers had asserted that tibolone is not aromatized in women and that the previous findings of 7α-methylethinylestradiol detection were merely a methodological artifact.[37][39][40] In accordance, a 2009 study found that an aromatase inhibitor had no effect on the estrogenic potencies of tibolone or its metabolites in vitro, unlike the case of testosterone.[5] In addition, another 2009 study found that the estrogenic effects of tibolone on adiposity in rats do not require aromatization (as indicated by the use of aromatase knockout mice), further in support that 3α-hydroxytibolone and 3β-hydroxytibolone are indeed responsible for such effects.[41] These findings are also in accordance with the fact that tibolone decreases sex hormone-binding globulin (SHBG) levels by 50% in women and does not increase the risk of venous thromboembolism (VTE) (RR = 0.92), which would not be expected if the medication formed a potent, liver metabolism-resistant estrogen similar to ethinylestradiol in important quantities.[1][42] (For comparison, combined oral contraceptives containing ethinylestradiol, due mostly or completely to the estrogen component, have been found to increase SHBG levels by 200 to 400% and to increase the risk of VTE by about 4-fold (OR = 4.03).)[43][44]

In spite of the preceding, others have held, as recently as 2011, that tibolone is converted into 7α-methylethinylestradiol in small quantities.[45][46] They have claimed that 19-nortestosterone derivatives like tibolone, due to lacking a C19 methyl group, indeed are not substrates of the classical aromatase enzyme, but instead are still transformed into the corresponding estrogens by other cytochrome P450monooxygenases.[38][45][46] In accordance, the closely structurally related AAS trestolone (7α-methyl-19-nortestosterone or 17α-desethynyl-δ4-tibolone) has been found to be transformed into 7α-methylestradiol by human placentalmicrosomesin vitro.[40][47] Also in accordance, considerably disproportionate formation of ethinylestradiol occurs when norethisterone is taken orally (and hence undergoes first-pass metabolism in the liver) relative to parenterally,[48][49] despite the absence of aromatase in the adult human liver.[46][50]

Progestogenic activity

Tibolone and δ4-tibolone act as agonists of the progesterone receptor (PR).[1][46][51] Tibolone has low affinity of 6% of that of promegestone for the PR, while δ4-tibolone has high affinity of 90% of that of promegestone for the PR.[1][46] In spite of its high affinity for the PR however, δ4-tibolone possesses only weak progestogenic activity, about 13% of that of norethisterone.[1][46] The weak progestogenic activity of tibolone may not be sufficient to fully counteract its estrogenic activity in the uterus and may be responsible for the 100 to 200% increased risk of endometrial cancer that has been observed with tibolone in women in large cohort studies.[1][46]

Androgenic activity

Tibolone, mainly via δ4-tibolone, has androgenic activity.[46][1] Whereas tibolone itself has only about 6% of the affinity of metribolone for the androgen receptor, δ4-tibolone has relatively high affinity of about 35% of the affinity of metribolone for this receptor.[46][1] At typical clinical dosages in women, the androgenic effects of tibolone are weak.[46][1] However, relative to other 19-nortestosterone progestins, the androgenic activity of tibolone is high, with a potency comparable to that of testosterone.[46][1] Indeed, the androgenic effects of tibolone have been ranked as stronger than those of all other commonly used 19-nortestosterone progestins (e.g., norethisterone, levonorgestrel, others).[46][1]